The present invention relates generally to touch sensor interfaces and more particularly, to fault detection in touch sensor interfaces.
Capacitive touch sensing enables touch detection by measuring a change in the capacitance of one or more touch detection electrodes. Since the human body is conductive, touch capacitive sensing technology can be used for human operated interfaces.
Referring now to FIG. 1, a schematic diagram illustrating a conventional touch sensor interface 100 is shown. The touch sensor interface 100 includes shared input/output (I/O) pads 102a and 102b, a touch sensing processor 104, sets of touch detection electrodes 106a and 106b, capacitors 108a, 108b, 108c, and 108d, communication channels 110a and 110b, and touch detection electrodes 112a and 112b. 
Each set of touch detection electrodes 106a and 106b includes one or more of the touch detection electrodes 112a and 112b. The capacitance of the touch detection electrodes 112a and 112b prior to a touch event comprises an external capacitance, viz. parasitic capacitance, Cext, corresponding to the capacitors 108a and 108c. After being touched, the capacitance of the touch electrodes comprises the parasitic capacitance Cext and a touch capacitance, Ctouch, between the PCB traces and external object (i.e., finger), corresponding to capacitors 108b and 108d. Thus, upon the occurrence of a touch event, the capacitance of a touch detection electrode at the touched location increases.
The touch sensing processor 104 scans the touch detection electrodes 112a and 112b for an increase in the touch capacitance. The scanning is accomplished using a touch sensing signal transmitted between the touch detection electrodes 112a and 112b and the touch sensing processor 104. The touch sensing signal travels over the communication channels 110a and 110b and the shared I/O pads 102a and 102b. The shared I/O pads 102a and 102b are associated with various I/O systems of the device to which the touch sensor interface 100 belongs, such as an illumination or visual source, an auditory source, a switch, a sensor, a haptics mechanism and the like. The shared I/O pads 102a and 102b are configured to operate in various modes corresponding to the different I/O mechanisms. For example, in a touch sensing mode, the shared I/O pads 102a and 102b are configured using software to monitor the touch detection electrodes 112a and 112b for a touch event. In an input mode, the shared I/O pads 102a and 102b are configured to monitor an input mechanism for the receipt of an input, and in an output mode, the shared I/O pads 102a and 102b are configured to provide an output to an output mechanism. In the touch sensing mode, the shared I/O pads 102a and 102b transmit the touch sensing signal to the touch detection electrodes 112a and 112b to measure an increase in the touch capacitance thereof. The touch sensing processor 104 processes the touch sensing signal to determine the occurrence of the touch event and generates a corresponding signal for the further operation of the device.
Since the touch sensor interfaces do not have moving parts, they are robust in design and thus do not require maintenance and have long operational lives. The technological suitability for human operation, the robustness of the design, and operational ease render touch sensor interfaces a preferred choice for human machine interfaces of various consumer electronic devices such as laptop computers, televisions, refrigerators, mobile phones, personal digital assistants and so forth. The touch sensor interfaces are also used in automobiles for gauges, door entry operation, windshield wiper switches, and other dashboard applications. Further, touch sensors may be used in automobiles for human safety critical applications such as seat belt alarm systems and air-bag deployment systems. Additionally, the touch sensor interfaces are used in critical medical equipment such as life support systems.
Although the touch sensor interface has a robust architecture, still they are prone to hardware and software faults such as a PCB trace open, a PCB trace short, a touch detection electrode leakage, a software configuration fault, and a communication channel short fault. Since, the touch sensor interfaces are used in applications related to human safety and life support systems, it is essential to detect the various hardware and software faults to ensure safe operation of the devices.